Long-Term Tillage and Straw-Management Effects on Soil Phosphorus Availability and Wheat Production Under Semi-Arid Dryland Monocropping

Long-term field trials are important for evaluating the long-term effects of crop and soil management practices on soil properties and productivity. The current study evaluated differences in soil organic carbon (SOC), total nitrogen (N), extractable phosphorus (P) and pH across tillage and straw-management systems of dryland wheat monocrop after approximately 40 years (1979–2018) of experimentation on an acric plinthosol. This study also determined the effects of tillage, straw management and N fertiliser rate on wheat grain yield after every 10 years, over 40 years. Treatments consisted of plots under conventional tillage (CT), stubble mulch (SM) and no-tillage (NT), combined with either straw burning (burnt) or retention (not burnt). Nitrogen fertiliser rates tested were 20, 40 and 60 kg N ha−1. Straw burning significantly increased soil pH and Bray-1 extractable P under NT, but not under other tillage systems. The Bray-1 extractable P declined down the profile, regardless of tillage or straw management. Total soil N was higher under NT than other tillage practices but did not vary with straw management, while SOC was not significantly affected by tillage or straw management. Wheat grain yield was significantly increased by higher N fertiliser levels and wetter seasons in recorded years. The results indicate that long-term NT was associated with greater soil N, while NT combined with straw burning was associated with higher soil pH and extractable P in the surface soil. However, the lack of significant differences in SOC among management treatments indicates that additional strategies may be required to increase SOC under this dryland wheat monocropping system. The observed variation in grain yield among production years points to the importance of water availability for wheat production under semi-arid conditions, although the effect of rainfall was not directly tested in the present study.